1. Field of the Invention
The invention relates to a variable reluctance pickup system for steel string musical instruments in which the vibrating strings cause variations of reluctance in a plurality of magnetic circuits generating electrical signals which, upon electronic amplification, are suitable for driving acoustic speaker systems.
2. Description of the Prior Art
Generally, variable reluctance pickup systems for steel string instruments comprise an arrangement of magnets and magnetically susceptible materials which establish a magnetic circuit in combination with the playing string. As the string vibrates, the changes in its position affect the reluctance and magnetic flux of the magnetic circuit. A sensing coil is inductively linked to the magnetic circuit for converting the variations in magnetic flux into a corresponding electrical signal. The electrical signal from the sensing coil is amplified electronically and fed into an acoustic speaker system for producing musical sounds.
There are many different configurations of the basic elements of variable reluctance pickup systems for steel string instruments. For example, U.S. Pat. No. 2,235,983 (Demuth) describes the basic elements of a magnetic pickup suitable for pianos and the like. U.S. Pat. No. 3,066,567 (Kelley) describes a magnetic pickup system having a single permanent magnetic element with a plurality of pedestals to provide a specific pickup zone for a given instrument string in combination with a single sensing coil. U.S. Pat. No. 3,483,303 (Warner) describes a variable reluctance transducer or pickup system for steel string musical instruments in which an attempt is made to isolate the magnetic circuits formed by adjacent strings so as to minimize "cross talk" between the various strings. U.S. Pat. No. 3,571,483 (Davidson) describes a variable reluctance pickup system having a plurality of isolated magnetic circuits, each specifically designed to be substantially insensitive to the plane of string vibration. Finally, U.S. Pat. No. 3,715,446 (Kozinski) describes a magnetic pickup system having a balanced coil assembly for each string wherein each assembly includes a bar magnet supporting two circular pole pieces and two sensing coils disposed around the pole pieces.
Before discussing the disadvantages of prior art variable reluctance pickup systems, it is instructive to review the fundamental properties of string instruments which give them their characteristic tones.
Basically, the tone of a plucked or struck string instrument is judged by the richness and complexity of the acoustic output in the "attack" or beginning portion of a note. In acoustic string instruments, the bridge structure constrains the motion of the sound board such that those components of string motion which are perpendicular to the plane of the sound board are well amplified, while those components of string motion which are parallel to the plane of the sound board are not. The path described by any arbitrarily small segment of a smoothly released plucked string is a precessing elliptical orbit of decreasing radius which rotates about the quiescent position of the string. Accordingly, the asymmetrical amplification of string motion provided by the bridge of an acoustic instrument yields a rich, full and complex tone of continuously varying harmonic content. The richness and complexity of the tone produced by acoustic string instruments is the primary criterion for judging the quality of such instruments.
Furthermore, the preferential or asymmetrical amplification provided by the bridge structure in acoustic string instruments enhances the expressive ability of the instrument. Specifically, the musician can control the initial motion of the string by plucking either parallel to the sound board for a "thin or nasal" tone or perpendicular to the sound board for a "full or rich" tone.
Steel string guitars and other similar instruments have a particular capability which distinguishes them from most other western musical instruments. This capability is referred to as "bending". Bending is accomplished after a string is fretted and plucked by moving the fretting finger with the string across the finger board stretching the string. The stretching of the string during bending can raise the pitch of the note played by as much as seven semitones, a factor which greatly enhances the expressive ability of the instrument. However, bending a note also results in a large displacement of the string from its quiescent position.
From the preceding discussion, it can be seen that for a variable reluctance pickup system to provide good tone (by acoustic instrument standards) it must be highly asymmetrical in converting string motion to electrical signal output. Further, such a pickup system must have a capability for high frequency response in order to preserve the richness and fullness of the varying harmonics in the attack portion of a note. Finally, for steel string guitars and similar instruments, the pickup system must be relatively insensitive to string displacements due to bending. Accordingly, variable reluctance pickup systems which are substantially insensitive to the plane of string vibration cannot generate a good acoustic tone.
Pickup systems with circular pole pieces have a similar disadvantage since the configuration of the magnetic field provided by such circular pole pieces is in the form of a symmetrical sinusoidal shell. Accordingly, the string vibrating within the magnetic field will generate equal magnitude electrical signals for string vibrations parallel to the string plane and string vibrations perpendicular to the string plane. The string plane is parallel to the sounding board.
Variable reluctance pickup systems which seek to eliminate cross talk between strings have two basic disadvantages: (1) elimination of cross talk also eliminates the possibility of bending; (2) isolation requires pole pieces having very narrow configurations aligned along the axis of the string, hence the pickup preferentially senses and generates electrical signals for string motions parallel to the string plane (parallel to the sounding board).
Other disadvantages of prior art variable reluctance pickup systems relate to their poor high frequency response characteristics. Specifically, the high frequency response of a magnetic circuit depends on two factors: (1) the aperture of the magnetic circuit (length of string sensed by the circuit); and (2) the impedance of the sensing coil.
Decreasing the aperture of the magnetic circuit increases its high frequency response capability. Specifically, the electrical signal response of a magnetic circuit reflects a summation of the changes of reluctance in the circuit which in turn are induced by movement of the linear portion of the string sensed by the circuit. If the length of string sensed is long (a large aperture), then different portions of the string within the aperture can move in opposite directions without changing the reluctance of the circuit. Hence, high frequency vibrational modes of the string may not be sensed for large aperture magnetic circuits. Accordingly, decreasing the aperture of the circuit increases the high frequency response of the circuit.
Variable reluctance pickup systems described in the prior art having "U-shaped" magnetic circuits have relatively large apertures. A U-shaped magnetic circuit comprises a bar magnet and two extending pole pieces positioned along an instrument string.
The impedance characteristics of variable reluctance pickup systems relate to the self-resonant nature of the sensing coils. Specifically, the impedance of the sensing coils increases with increasing frequency up to a maximum at the resonant frequency, whereupon the impedance decreases. Below the resonant frequency, the impedance is dominated by inductive effects. Specifically, the vibrating string causes variations in the magnetic flux of the magnetic circuit. The resulting variations in magnetic flux in the vicinity of the sensing coil induces an electrical signal in the coil which, in turn, creates another magnetic field which "bucks" or opposes the variations in flux induced by the string (Lenz's Law). This effect "impedes" the signal and increases with increasing frequency.
Above the resonant frequency, the impedance is influenced by the capacitive effects between turns of the coil and between layers in the coil winding. Specifically, the changing current in one turn of the coil influences the current in the neighboring turns of the coil. This effect becomes larger with increasing frequency such that the coil behaves as a capacitive reactance with turn-to-turn capacitive leakage to ground. Accordingly, the output signal from the sensing coils falls off rapidly above the self-resonant frequency.
Since both the inductance and capacitance of a sensing coil vary linearly with its mean radius, replacing one coil by multiple small coils can reduce the impedance of the pickup system by a factor equal to the number of coils and raise the self-resonant frequency by a factor equal to the square root of the number of coils.
From the above discussion, it can be seen that prior art variable reluctance pickup systems having a single coil for sensing variations in the reluctance of the magnetic circuit will have poor high frequency response. The desirability of utilizing multiple sensing coils to enhance the high frequency response characteristics of variable reluctance pickup systems, while recognized in the prior art, has not been extensively utilized. For example, in Kozinski, each pole piece has a separate sensing coil, which coils are connected in series to reduce background noise due to electromagnetic fields (a conventional humbucking arrangement). However, the Kozinski pickup is extremely sensitive to string position and cannot generate a sustained and continuous output upon bending a string from its quiescent position.